cc: Trace the viewport size changing.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 53eb58bb0c4a19007d295abd92a9db09fcff7201..708970213396fa02dcb6ce2620b92c8da447c2cc 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2198,6 +2198,10 @@ void LayerTreeHostImpl::ReleaseGL() {
 void LayerTreeHostImpl::SetViewportSize(const gfx::Size& device_viewport_size) {
   if (device_viewport_size == device_viewport_size_)
     return;
+  TRACE_EVENT_INSTANT2("cc", "LayerTreeHostImpl::SetViewportSize",
+                       TRACE_EVENT_SCOPE_THREAD, "width",
+                       device_viewport_size.width(), "height",
+                       device_viewport_size.height());
 
   if (pending_tree_)
     active_tree_->SetViewportSizeInvalid();